Simplified schematic of the Y-shaped pipeline system linking the Poseidon fields to the onshore gas treatment plant.
Spain's flagging oil production sector has been boosted by a new discovery in the western Mediterranean. Operator Repsol-YPF is keen to bring the small Chipiron Field onstream in 2001, and also plans renewed gas field activity in the Gulf of Cadiz and the Cantabrian Sea.
Chipiron is located in the Gulf of Valencia, seven km west of the Casablanca oilfield. A production platform exports oil from this and other satellite fields northwards to the Repsol-YPF refinery at Tarragona via a 48 km, 14-in. pipeline. The discovery well was drilled to a total vertical depth (TVD) of 3,388 meters and intersected a net interval of 70 meters in the Lower Tertiary and Lower Cretaceous. On test, it flowed 7,000 b/d of 40° API crude through a 1-in. choke.
Repsol-YPF hopes the new discovery will allow it to nearly double the current production from this area to over 12,000 b/d. That would still leave plenty of spare capacity in the pipeline, which is sized for 50,000 b/d throughput. The field will be developed through a single subsea wellhead connected to the Casablanca platform via a 10-km flowline. Front-end engineering design (FEED) work has been conducted in-house, with a view to achieving first output from Chipiron early next year.
The oil is of good quality, although paraffin formation is thought to be a potential problem. This will likely be resolved by pigging. Seabed conditions (mud overlying hard rock) also present difficulties for the flowline installation.
Chipiron's reserves are estimated at six million bbl, compared with 4-5 million bbl for the other producing satellites. Of these, Rodaballo, was developed through another subsea completion and 10 km flowline, while Boqueron flows through a 45° extended reach well drilled from the platform and completed in 1997. Aside from these fields, all drilled by Repsol over the past five years, there has been one sub-commercial discovery, testing 300 b/d.
According to Ramon Hernan, an exploration and production managing director at Repsol-YPF, the company plans to explore two other prospects from the Casablanca platform - which has four unused well slots - using its own tender-assist drilling rig. Subsea completions or extended reach wells are favored solutions for future discoveries, but the company may also consider a floating production, storage, and offloading (FPSO) vessel for more remote finds.
Repsol-YPF works with four partners in various licences in this area - CNWL Oil (Espana), an offshoot of Canada's Sherritt International, Dutch company Oranje-Nassau, and Spanish companies CIEPSA and LOCS. "We are always drilling the same model," says Hernan, "which is the Mesozoic karst. Another oil-bearing formation is the Basal Tertiary Group, which are dolomite breccias deposited on the karstified paleohighs. The overlying Casablanca formation is the main source rock. The petroleum system is very well known."
The operator was less successful last year with another exploratory well drilled in the Tortuga concession south of Chipiron. According to analysts Wood Mackenzie, numerous wells had been drilled across this acreage by previous operator Shell, including its depleted Tarraco/Castellon Field, which produced 14.6 million bbl of oil during 1977-86. Tortuga-1 was spudded to the northeast of this field last March and drilled to a total depth of 3,450 meters. Following suspension, it was re-entered last June. A total of 1,129 bbl of 40° API good quality oil was recovered from an open-hole DST, but no commercial flows were produced. The well was abandoned.
Cantabrian Basin re-visited
Map shows location of Chipiron discovery in relation to other fields in the Casablanca production complex, offshore Tarragona.
Offshore the northern Spanish city of Bermeo, Repsol-YPF hopes to resume exploration drilling in 2001-02, in association with Murphy, Wintershall, and local company SHESA. Repsol acquired new 3D seismic over this region in late 1998, and is currently concluding the interpretation. Most of the existing datasets were assembled in the 1970s and 1980s.
Water depths over the prospective areas range from 40 meters in the south to 160 meters in the east. Repsol produced gas from its offshore licences here for over 10 years, initially from the unmanned Gaviota platform, which was installed in the mid-1980s. In 1994, it tied in the Albatross satellite via a subsea completion and a 17.5 km, 8-in. dia. pipeline and 400 meter flexible riser. The pipeline was connected to the wellhead through a double "Y" manifold and 150 meter flexible jumper.
Since production from this complex ceased a few years ago, the facilities have been used as underground storage for excess gas from the trans-Iberian Enagas pipeline. But the complex could resume its former role if new discoveries were made - both the platform and subsea equipment are still in-situ. In that case, it would alternate between a storage and production mode for several months at a time. Alternatively, Repsol-YPF could tie any new discoveries directly to the shore, on the lines of its more recent development in the Gulf of Cadiz.
Offshore Huelva in Andalucia, Repsol-YPF has produced gas from the Poseidon fields since early 1998. This has been the company's most ambitious development to date off mainland Spain, involving Europe's second longest subsea tieback (after Norsk Hydro's TOGI).
Exploration drilling in the Gulf of Cadiz started in 1965, with 27 wells drilled over the next two decades by Shell and Campsa in water depths of 60-500 meters. No commercial successes were recorded until the 1980s, when seven small gas fields were discovered. These low volumes, allied to gas prices at the time, ruled out a conventional platform-based solution for a development.
Studies continued, nevertheless, into the early 1990s when advances in subsea technology began to open up other avenues for operator Repsol. These included long-distance control of the wells directly from a shore-based location, using an electro-hydraulic umbilical and subsea trees. This still left other obstacles to surmount, such as how to produce from unconsolidated sandy formations and the laying of a subsea pipeline in very soft mud.
Repsol eventually settled on a scheme focusing on the two largest reservoirs, Poseidon North and South, which lie in respective water depths of 78 meters and 132 meters. Their combined reserves are put at 1.7 bcm. The gas would be piped 65 km to an onshore gas treatment plant before feeding into the Enagas national grid.
During 1996-1997, three development wells were drilled on the two fields by the South Seas Driller semisubmersible. To avoid formation damage, an easily washable, self-saturated fluid was deployed for the completions, and in place of a standard liner, 7-in. sand control filters were installed to limit sand production. Control signals are transmitted to the concentric, 5,000 psi trees via an umbilical that also incorporates hoses for injecting hydrate inhibitors as well as hydraulic fluid for the valve actuators.
Produced gas is exported through a Y-type 10-in. diameter pipeline system installed by the Castoro 8 (the central manifold is located at the junction, 24 km from the shore). The pipeline extends inland for a further 7 km to the gas treatment plant, based on the outskirts of Donana Nature Reserve. This plant, which has treatment capacity for up to 1.5 MMcm/d, also compresses the gas when its pressure drops below 70.6 bar.
The seabed along the route of the pipeline and the subsea structures is very soft clay. This necessitated protection of the pipeline by a 2.5-meter-deep trench dug by Allseas' Trenchsetter vessel. The caterpillars on the Digging Donald trenching machine also had their bearing surface increased to counter problems anticipated with the soil-bearing capacity. For the shore approach, water jetting was employed to remove the sands, which were then sucked up and deposited on the side of the trenches. Subsequently, these were refilled with shales to prevent upheaval buckling of the pipeline.
Both Poseidon reservoirs are expected to start depleting by 2001. Repsol-YPF may decide to sustain volumes by developing the remaining, smaller discoveries, according to Hernan. Poseidon North may later be converted for use as gas storage.
Editor's Note: A previous version of this article appeared as a paper in the Offshore Mediterranean Conference 1999, held in Ravenna, Italy.